Specific substance properties, e.g., the carbon dioxide concentration of respiratory air, the humidity of wood or paper, or the composition of plastics may often be analyzed by means of comparatively simple optical methods, e.g., with the aid a polychromator. In a large amount of application cases, comparatively simple information may be used for controlling an operating sequence or a process. Controlling drying of raw paper or determining the colorific value of wood, or wood pellets, are based on determining humidity. Just like determining the content of carbon dioxide in air, spectral-analytic system may provide accurate measurement values in said application cases merely by evaluating only two spectral bands—a measurement band and a reference band.
What is typical for said applications and many other examples of applications is the critical cost situation regarding the system. Low-cost systems make a decisive contribution to keeping manufacturing costs low. What is relevant is the respective total cost of ownership (TCO), which includes service and operating costs. Often, there are solutions which are technically feasible but involve too much expenditure in the long term. For example, commercially available near-infrared spectrometers are problematic because of their high investment costs; other approaches based, e.g., on optical filters or LED light sources are often limited in terms of reliability or long-term stability.
What is desirable is a system approach which is characterized by a small amount of expenditure in terms of manufacturing and operation and performs simple spectral-analytical measurement with reliability and long-term stability. The selection of the spectral bands considered should be easily adaptable within the context of the manufacturing process, the level of variability should be as large as possible, and the overall solution should be small, robust and low-cost.
Conventional technology discloses numerous methods of detecting chemical composition. Problems provided in a gaseous, liquid or dissolved form may be analyzed by means of chromatographic methods. Measurement is generally destructive.
Radiographic methods, e.g., X-ray florescence analyses (XRF) or atomic absorption spectrometry (AAS) involve a large amount of expenditure and represent potential health hazards.
Optical spectroscopy is a widely employed method both for utilization in laboratories and for performing field measurements. Complex spectral-analytical measurement tasks are typically performed by using spectrometers. Said spectrometers are available in manifold variants and for various spectral ranges. However, specifically within the range of near-infrared and infrared wavelengths, which range is important for analyzing organic compounds and water, spectrometers are expensive and often too sensitive for being used in the field and in production. Miniaturized variants of spectrometers, which are mainly based on designs having fixed gratings and detector lines, may reduce said disadvantage but are still too expensive for many applications.
System approaches based on interferometers, so called Fabry-Perot filters, are often critical with regard to vibrations occurring during use. Other approaches using spectral filters exhibit disadvantages regarding reliability and long-term stability. This also applies to approaches wherein light of different wavelengths is generated, for example, by selected LEDs.
Conventional technology also describes so-called polychromators which, similar to spectrometers, split up incident light into its spectral constituents but then will detect said incident light only at selected points of the spectrum in that a single detector is positioned at the appropriate location behind a suitably configured gap for aperture limitation. Such systems have so far been used mainly in the field of very high resolutions with very large designs. Miniaturization has been limited due to the adjustment expenditure which has so far been involved in order to achieve the desired level of precision.